After a well, completed into a formation, has been producing oil or gas over an extended period of time, the rate of production generally diminishes, often due to depletion of the reservoir or due to near-wellbore effects. Methods of alleviating diminished production can include treating the near-wellbore effects and increasing the drainage area or wellbore access. Treatment of near-wellbore effects include hot oil flushing to melt paraffins, high pressure fracturing, chemical treatments, or re-perforation of the casing and acidizing to open up additional flow passages. Each of these treatments are subject to restrictive use or success of short duration.
A more progressive solution is to increase the drainage area. This is generally accomplished by drilling holes laterally outwardly from the wellbore so as to increase communication with the formation. These holes are known as drainholes.
Typically, the hydrocarbon bearing portion of the formation is rather shallow. This delimits where the lateral drainholes are placed, requiring significant precision in vertical placement. Additionally, the drainholes must first pass through the existing casing and then extend into the formation.
Whipstock diversion or horizontal drilling techniques using mud motors account for most of the re-entry drilling techniques. Generally a full drilling rig is required and is used in combination with a whipstock to deviate the drill string. A portion of the casing is milled out and a rotary drilling string or mud motor essentially drills a new wellbore. This requires a large radius of turn which complicates targetting of the payzone. The process is expensive and results in a single new hole.
Lance-type penetrators, such as that disclosed in U.S. Pat. No. 5,392,858 to Peters, introduce apparatus to first mill through the casing and then provide a flexible conduit which supplies high pressure fluid to a nozzle. The nozzle jets forward while advancing, hydraulically cutting into the formation. Small radii (12") can successfully be achieved. Unfortunately, the high pressure fluid can erode the casing cement and re-establish undesirable cross-communication with vertically adjacent layers.
A lesser known technique is to provide a section of highly flexible drill shaft at the downhole end of a rotary shaft. These techniques use a single coiled spring as the power transmitting member with an internal or external elastomer sheath or hose to contain drilling fluids. These systems, as disclosed in U.S. Pat. Nos. 3,838,736 and 4,051,908 to Driver, have the following features in common: a tubing string is lowered into the casing, the string having a 90 degree elbow at its lower end; a flexible hollow shaft is connected to the lower end of drill pipe and is lowered down into the tubing string; the drill string is rotated, the flexible shaft is directed laterally by the elbow and proceeds to drill through the casing and into the formation. These and similar systems are limited to low drilling rotational speeds and low axial loading to avoid premature failure of the coil spring flexible shaft.
In the context of stabilizing the roofs of mines, a flexible drill shaft is used to drill holes upwardly into the roofs. By providing a flexible shaft, shaft lengths and thus hole depths greater than the height of the mine corridor can be achieved. As disclosed in U.S. Pat. No. 4,057,115 to Blanz, contra-wound bands or springs are used for the shaft. An outer band is helically wound about a coil spring having an opposite pitch. A drill bit is secured to the shaft's upper end. A rotary drive clamps onto the circumference of the outer band and applies torque. The drive and shaft are advanced axially upwardly, driving the bit into the mine's roof. When the rotary drive approaches the roof, it is unclamped, lowered axially and is re-clamped onto the shaft. During drilling, the outer band tends to contract, and the inner coil tends to expand, lending axial stability to the shaft.
This apparatus does not address the difficulties of downhole operation, including the ability and the need to introduce an axial load into the flexible shaft yet still make small radius turns, wherein the axial load originates before the turn is made.